The present invention relates to apparatus for use as a borescope and in particular to an orbital scanning borescope. Such borescopes are commonly used for inspection of gas turbine engines to determine if there are any cracks or defects in components such as turbine blades. It is known in this field to incorporate a reticle (or graticule) in the borescope which allows a user to estimate the size of a crack or defect which is viewed through the scope, in order to determine whether the engine is airworthy.
Typically, these reticles have been fixed in the borescope in two ways. Firstly, the reticle may be fixed in position in relation to the handle/control section of the proximal end of the borescope. This means that when a borescope is orbitally scanned the reticle does not rotate with the image but stays in a fixed orientation. Secondly, the reticle may be fixed in relation to the insertion tube containing the lens system. In this version, when the borescope is orbitally scanned, the reticle always rotates with the image.
Both of these approaches present certain problems in that it may not be possible to correctly align the reticle with the feature to be measured, making the measurement far more difficult.